atomic_t variables are currently used to implement reference counters with the following properties: - counter is initialized to 1 using atomic_set() - a resource is freed upon counter reaching zero - once counter reaches zero, its further increments aren't allowed - counter schema uses basic atomic operations (set, inc, inc_not_zero, dec_and_test, etc.)
Such atomic variables should be converted to a newly provided refcount_t type and API that prevents accidental counter overflows and underflows. This is important since overflows and underflows can lead to use-after-free situation and be exploitable. The variable table_device.count is used as pure reference counter. Convert it to refcount_t and fix up the operations. Suggested-by: Kees Cook <[email protected]> Reviewed-by: David Windsor <[email protected]> Reviewed-by: Hans Liljestrand <[email protected]> Signed-off-by: Elena Reshetova <[email protected]> --- drivers/md/dm.c | 12 +++++++----- 1 file changed, 7 insertions(+), 5 deletions(-) diff --git a/drivers/md/dm.c b/drivers/md/dm.c index 4be8532..be12f3f 100644 --- a/drivers/md/dm.c +++ b/drivers/md/dm.c @@ -24,6 +24,7 @@ #include <linux/delay.h> #include <linux/wait.h> #include <linux/pr.h> +#include <linux/refcount.h> #define DM_MSG_PREFIX "core" @@ -98,7 +99,7 @@ struct dm_md_mempools { struct table_device { struct list_head list; - atomic_t count; + refcount_t count; struct dm_dev dm_dev; }; @@ -685,10 +686,11 @@ int dm_get_table_device(struct mapped_device *md, dev_t dev, fmode_t mode, format_dev_t(td->dm_dev.name, dev); - atomic_set(&td->count, 0); + refcount_set(&td->count, 1); list_add(&td->list, &md->table_devices); + } else { + refcount_inc(&td->count); } - atomic_inc(&td->count); mutex_unlock(&md->table_devices_lock); *result = &td->dm_dev; @@ -701,7 +703,7 @@ void dm_put_table_device(struct mapped_device *md, struct dm_dev *d) struct table_device *td = container_of(d, struct table_device, dm_dev); mutex_lock(&md->table_devices_lock); - if (atomic_dec_and_test(&td->count)) { + if (refcount_dec_and_test(&td->count)) { close_table_device(td, md); list_del(&td->list); kfree(td); @@ -718,7 +720,7 @@ static void free_table_devices(struct list_head *devices) struct table_device *td = list_entry(tmp, struct table_device, list); DMWARN("dm_destroy: %s still exists with %d references", - td->dm_dev.name, atomic_read(&td->count)); + td->dm_dev.name, refcount_read(&td->count)); kfree(td); } } -- 2.7.4
